Fibromyalgia (FMS) and chronic fatigue syndrome (ME/CFS) are difficult illnesses to classify. Both have a broad range of physical symptoms that span multiple systems, and they're associated with multiple psychological symptoms as well. In addition, they're often accompanied by a slew of other illnesses — many of which are also hard to classify.
As scientists are getting more of a handle on FMS, ME/CFS and other related illness, an umbrella term that's more frequently used to describe them is central sensitivity syndromes, or CSS. Some researchers argue that this term should replace other terms, such as functional somatic syndrome, medically unexplained syndrome, and somatoform disorders, because they believe CSS is more accurate.
An illness described as a CSS involves central sensitization. "Central" refers to the central nervous system, which is made up of the brain and spinal cord. "Sensitization" describes the end result of a process that leaves someone sensitive to a particular kind of input.
Allergies are the type of sensitivity people are generally the most familiar with, as they involve an inappropriate physical response to something that other people can tolerate just fine. While the sensitivities of a CSS don't cause the same physical reaction as an allergy, they do trigger an inappropriate physical response.
In a CSS, we become sensitive to the things that are processed by the central nervous system, which can include bright lights, loud noises, strong smells, rough textures, and pressure on the body. It may also involve certain foods or chemicals. Especially in FMS, the body is sensitized to anything unpleasant, called "noxious stimuli" by researchers.
Aside from FMS and ME/CFS, the following conditions have been proposed to be part of the CSS family:
Psychiatric disorders are common in CSS as well. Research suggests that's because they all involve dysregulation of the same neurotransmitters, with the dysregulation in CSS in different regions of the brain than in psychiatric disorders.
Psychiatric conditions that commonly overlap with CSS include:
Neurotransmitters that are involved in at least some CSS include:
The pain of CSS comes from a couple of different abnormal pain types: hyperalgesia and allodynia.
Hyperalgesia takes normal pain from things that everyone considers painful (a broken limb, an infected tooth, etc.) and makes it worse. It's often referred to as "turning up the volume" of pain. This makes things like injuries, surgeries, and chronic sources of pain especially debilitating.
Allodynia makes you feel pain from things that shouldn't hurt, such as the brush of fabric against your skin, or your arm resting against your side when you sleep. Allodynia can make your clothes painful even when they're not too tight, or make you unable to enjoy a hug. It turns all manner of ordinary experiences into painful ones, which can cause people to make significant changes to their lives to minimize it.
Other proposed mechanisms of CSS include:
The different individual symptoms and mechanisms of each CSS require a customized treatment approach, but in general, most CSS tend to respond to some of the same types of treatment, especially antidepressants (which help correct neurotransmitter dysregulation),exercise, and cognitive behavioral therapy (CBT).
However, it should be noted that people with ME/CFS have special considerations when it comes to exercise, and CBT is a highly controversial treatment for this illness, especially when it involves graded exercise.
The resources below can help you learn more about individual CSS. Because of the similar symptoms they can have, it's important that each one is diagnosed by a doctor.
Chong YY, Ng BY. Annals of the academy of medicine, Singapore. 2009 Nov;38(11):967-73. Clincal aspects and management of fibromyalgia syndrome.
Mayer TG, et al. Pain practice. 2012 Apr;12(4):276-85. The development and psychometric validation of the central sensitiz...
Smith HS, Barkin RL. American journal of therapeutics. 2010 Jul-Aug;17(4):418-39. Fibromyalgia syndrome: a discussion of the syndrome and pharmacothe...
Smith HS, Harris R, Clauw D. Pain physician. 2011 Mar-Apr;14(2):E217-45. Fibromyalgia: an afferent processing disorder leading to a complex ...
Yunus MB. Seminars in arthritis and rheumatism. 2008 Jun;37(6):339-52. Central sensitivity syndrome: a new paradigm and group nosology for...
Yunus MB. Seminars in arthritis and rheumatism. 2007 Jun;36(6):339-56. Fibromyalgia and overlapping disorders: the unifying concept of cen...